This project correlates behavioral responses with neural responses of thalamic projection and non-projection neurons in the medullary dorsal horn (trigeminal nucleus caudalis) produced by noxious thermal stimuli in the behaving monkey. Medullary dorsal horn neurons encode thermal discriminative information used by the monkey to perform a thermal detection task. Many medullary dorsal horn neurons encode thermal intensity in a manner which allows the detection of small changes in noxious temperatures. Certain subpopulations of medullary dorsal horn neurons appear to encode thermal intensity information better at near threshold detection levels than order thermally responsive medullary dorsal horn neurons. In addition, some thermally sensitive neurons also respond to other stimuli used by the monkey for the successful completion of the task. This task-related activity occurs in characteristic patterns of excitation and/or inhibition and some neurons which exhibit such activity project to the thalamus. The neurons examined in this study appear to be important in encoding noxious sensory discriminative information. The task-related responses exhibited by some of these neurons may modulate sensory activity and thereby influence the perception of and response to oral-facial pain. In addition, this project studies the ability of rhesus monkeys and humans to detect small changes in innocuous warm pulses and noxious heat pulses applied to the face and the influence of attention on thermal discriminative capacity. A signal correctly indicating the location or modality of a subsequent thermal change improves detection performance while an incorrect signal worsens performance. Attentional manipulations have a greater influence on the ability to detect changes in noxious heat as compared to innocuous heat. Consequently, attentional factors may influence the perception of and response to oro-facial pain.